The present invention relates to a method and apparatus for producing coal-water fuel (CWF) on a commercial scale which uses a unique application of conventional, commercially available equipment.
Individual unit operations in the invention include coal crushing, rod milling, sieve bend screening, froth flotation, vacuum filtration, refuse dewatering, and ball milling. These have been practiced in the coal preparation and minerals beneficiation industries for many years. The invention also uses a reverse flotation operation.
The size reduction unit operations; crushing, rod milling, and ball milling, are common in mineral processing plants, e.g. copper and molybdenum ore concentration operations. Rod and ball milling are not found in conventional coal beneficiation operations. Current practice is to avoid the production of fine coal, primarily because of the inefficiency of conventional fine coal cleaning operations.
In conventional coal froth flotation, chemical reagents are added to the pulverized coal-water mixture to permit air bubbles to selectively attach to coal particles, causing them to rise to the surface. The particles of mineral matter remain on the bottom of the flotation cell. For reverse flotation, a different chemical reagent package provides for depression (sinking) of the coal particles and selective attachment of air bubbles to particles of liberated pyrite. Thus, pyrite, which is the principal sulfur-containing mineral associated with coal, rises to the surface and can be skimmed off, resulting in a reduction of the sulfur content of the feed stream.
Froth flotation is a commercially proven technique for reducing the ash content of the feed coal. In most conventional coal flotation applications, only ten to twenty percent of the total plant feed is passed through the flotation circuit. In the present invention the entire feed stream may be directed to the flotation circuit depending on coal characteristics. Separation of the flotation feed into coarse and fine streams (split feed) has been demonstrated to improve the performance of the flotation circuit. Several commercial operations do practice split feed flotation, but this is not common. Separate flotation of coarse coal was first performed about 1960 at the pipeline plant of Hanna Coal Company in Cadiz, Ohio. The Kerr McGee Company has also installed split feed flotation for processing 28 mesh.times.0 raw coal in their newest 1200 TPH preparation plant. Multiple stage or "rougher-cleaner" flotation has been practiced in the coal industry for over 20 years. The first rougher-cleaner circuits in the coal industry in the U.S. were designed and installed in 1963 at three plants of Bethlehem Mines Corporation in Washington County, Pa. The rougher-cleaner flotation circuits were desiged for 60 TPH of 28 mesh.times.0 coal.
The reverse flotation process has been tested at both the laboratory and pilot plant levels (12 TPH coal feed) on a number of Pennsylvania and West Virginia coals. These tests indicate that 70% to 90% of the pyritic sulfur could be removed by reverse flotation. Much of the early work, beginning in the late 1960's, was supported by the U.S. Bureau of Mines. Technical details of the process are available in the literature. Several individual companies continued this work in privately sponsored research programs.
The use of a vacuum disc filter for dewatering of fine particles is common practice in both coal beneficiation and minerals processing plants. The present invention, however, requires more sophisticated control than commonly found in existing coal cleaning plants. However, such sophisticated control is standard practice in iron ore benefication systems where filter cake moisture is a crucial parameter in the subsequent pelletizing operation.
The final stage in the CWF production process of the present invention, high density ball milling, has been demonstrated at a pilot scale. A 50 to 100 TPD continuous pilot plant located at Kennedy Van Saun Corporation in Danville, Pa., has been in operation since February 1982. The coal-water fuel technology is covered in U.S. Pat. Nos. 4,282,006 and 4,441,887 to Funk.